Photodetectors for infrared (IR) radiation have many applications. IR radiation can often be used to detect objects where visible light is either blocked or not present. Thus, it is possible to use IR detection at night or through clouds, dust, or haze. An IR photodetector can be in the form of either a single detecting element or an array of such detecting elements to produce an image. A photodetector that produces such an image is termed a focal plane array.
IR radiation can be detected by various methods. One such method is using a material that generates electric charge carriers in response to absorbing incident IR radiation. Examples of such photosensitive material are mercury cadmium telluride (HgCdTe) and a multi-layered structure termed a multiple quantum well (MQW) structure that has alternating layers of wells and barriers. Photodetectors fabricated from MQW material are termed quantum well IR photodetectors (QWIPs).
Depending upon the application, photodetectors may place great importance on sensitivity. Many different photodetector designs have been promulgated in the effort to satisfy this need for high sensitivity. Most of these designs, however, place very stringent requirements upon the materials used in the photodetector and/or the processes by which the photodetectors are fabricated. The result is that these photodetectors are expensive. Thus, there exists a need for a photodetector design that relaxes the material and/or fabrication process requirements, thereby reducing costs.
Still other applications require a photodetector that is sensitive to more than one band of infrared radiation, i.e., multi-color photodetectors. Such applications place even more stringent requirements on the materials and the fabrication process than do single color photodetectors. Further, many of the high sensitivity designs for single color photodetectors cannot be employed in multi-color photodetectors without significantly reducing sensitivity. Thus, there exists a need for a photodetector design that can be applied to both single and multi-color photodetectors without reducing the sensitivity of the photodetector.